This invention relates to a method of welding turbine diaphragms, and, more particularly, to a method of joining spacers, provided with a plurality of nozzle blades, to outer and inner wheels by welding.
A turbine rotates a rotor carrying thereon a plurality of turbine blades, with thermal energy of motive fluid such as steam being received by the turbine blades. In order to guide the motive fluid, diaphragms with a plurality of nozzle blades arranged annularly are disposed upstream of the turbine blades and supported by a turbine casing. The diaphragms each comprise outer and inner wheels and spacers with the plurality of nozzle blades. The spacers are joined to the outer and inner wheels by welding.
In order to weld between the spacers and the outer and inner wheels, various welding methods have been proposed. In one proposed method the spacers and the inner and outer wheels are welded by arc-welding, with weld beads being formed on the whole interface between the spacers and the outer and inner wheels except for recess portions formed in the spacers at the central portions of the nozzle blades. Another welding method is described in Japanese Laid-open patent application No. 53-30436 (1978), wherein an electron beam welding method is proposed for welding along the whole intefaces between spacers and the wheels except for central parts of the blades. Thus, the proposed conventional welding method each apply welding along the whole deep interfaces formed in contact relation between the spacers and the outer and inner wheels. As for welding defect occurrence rate, there is a tendency that the welding defect occurrence rate increases as weld depth increases. In the conventional welding methods, weld is applied along the deep interface; therefore, a joined portion by the welding method has a large welding defect occurrence rate, wherein the defects include cracks, blow holes, weld bead deflection from the interface to be welded, etc.
Further, in the arc-welding method, it is necessary for such a deep weld depth to form a large V-shaped groove; therefore, a large amount of weld beads are necessary, which results in increase in labor for welding and an increase in thermal deformation of the diaphragm.
In case where electron beam welding is applied so as to form deep weld, the above-mentioned type of defects are apt to occur. Further, the electron beam welding has difficulty of positioning an electron gun of an electron beam welding machine in alignment with the interface of material to be welded.